The present invention pertains to ultraviolet light stabilizers for plastics and coating compositions, or more particularly, to novel types of compounds which are asymmetrical benzotriazolylphenols.
It is well known in the art that plastics are unstable over time upon exposure to ultraviolet light. Such plastics undergo a decrease in mechanical strength and an increase in unwanted color when exposed to uv radiation. It is also known to improve the resistance of plastics and coating compositions to deterioration by the addition of a uv light absorber. Such commonly used stabilizers include compounds of the hydroxyphenyl benzotriazole family, including hydroxyphenyl benzotriazole monomers and dimers. Hydroxyphenyl benzotriazole monomers are well known in the art and are disclosed in earlier U.S. Pat. Nos. 3,204,896; 3,189,6515; 5,097,041; 4,943,637 and 5,104,992. Hydroxyphenyl benzotriazole dimers are also known.
The benzotriazole dimers have an advantage over the monomeric benzotriazole u.v. absorbers. The dimers have higher molecular weight which makes them much less volatile. This is very important considering the high temperatures required for the processing of engineering plastics. In addition, these dimers possess high thermal stability so they are not decomposed when exposed to elevated temperatures for prolonged periods of time.
Benzotriazole type u.v. absorbers, including dimers, function through a mechanism involving the phenolic hydrogen and the nitrogens of the triazole ring. In attempts to reduce the volatility of benzotriazole monomers, various organic groups were added to the phenolic ring and/or halogens to the benzene ring of the benzotriazole. Volatility was usually reduced by substitution of one or two organic groups on the phenolic ring. While these measures did improve the volatility problem, it also reduced the total u.v. absorption capability of the benzotriazole monomers. Most commercial benzotriazoles which are available today are disubstituted.
U.S. Pat. Nos. 4,684,679, and 4,812,498 disclose the symmetrical dimers 2,2'-methylene-bis-(4-hydrocarbyl-6 benzotriazolylphenols). U.S. Pat. Nos. 4,684,679 and 4,937,348 disclose a process for preparing the symmetrical dimers 2,2'- methylene-bis-(4-hydrocarbyl-6-benzotriazolylphenols) by first forming the Mannich base of a described benzotriazole monomer and then reacting the Mannich base with the same benzotriazole monomer or the same Mannich base. All of the above patents are incorporated herein by reference. Indeed, a common feature of all known benzotriazole dimers is that they are symmetrical, that is, all units of the dimer or polymer contain the same single hydroxyphenyl benzotriazole monomer unit. For example, U.S. Pat. No. 4,937,348 shows a hydroxyphenyl benzotriazole dimer having the formula: ##STR2## These compounds are symmetrical because each R.sub.2, which is an alkyl group having 1-18 carbon atoms at the 4 position of each of the phenolics, is identical. To synthesize benzotriazole dimers, one is limited to benzotriazole compounds where there is but a single substitution on the phenolic ring. Disubstitution on the phenolic ring prevents the formation of a dimer through a Mannich base or by any known reaction. This fact limits the number of benzotriazole dimers to the few monosubstituted benzotriazoles commercially available.
This is disadvantageous because the physical properties of symmetrical dimers of hydroxyphenyl benzotriazoles have limited properties with regard to melting point, molecular weight, uv absorption distribution, particle migration, and polymer compatibility. Since there are so few compounds in this dimer group, it is often impossible to select a symmetrical hydroxyphenyl benzotriazole having the best combination of properties. For example, while it may be desired for a benzotriazole to have a certain uv absorption characteristic or melting point, it may not be compatible with the plastic of choice. It has now been unexpectedly found, that by independently varying the alkyl (hydrocarbyl) group at the 4-position of the phenolic ring so that each half of the dimer has a different substitution, numerous dimers can now be synthesized. The melting point, molecular weight, and uv absorption distribution properties can be tailored to the requirements for the polymer of choice. It has been found that each dimer half contributes its beneficial characteristics. Also unexpectedly, the melting points of the asymmetrical dimers are lower than one would normally expect. This allows the asymmetrical dimers to be used with polymers which are processed at lower temperatures and the lower melting point allows easier dispersion in polymers.
It has also been unexpectedly found that in general, asymmetrical dimers display higher solubility in organic non-polar solvents than symmetrical dimers made from the same monomers, as shown in Table 1 below. The symmetrical dimers usually have limited solubility in polar and non-polar solvents and have higher melting points. With better solubility and lower melting point, asymmetrical benzotriazoles are more easily and more uniformly blended in the plastic to be stabilized. The asymmetrical benzotriazoles make it possible to synthesize many different combinations of monomers to form dimers with tailor made characteristics.